Cluster studies of the interaction of oxygen with the lithium (100) surface

The interaction of an oxygen atom with the lithium (100) surface is studied using cluster models. On the basis of ab initio Hartree-Fock—linear-combination-of-atomic-orbitals theory the electronic structure of ${\mathrm{Li}}_n$ and ${\mathrm{Li}}_n\mathrm{O}$ clusters ($n\le 9$) is calculated for the three high-symmetry surface positions of the oxygen: on top, central, and bridge positions. The geometry of the Li centers in the clusters is chosen to represent a section of the bcc crystal at the (100) surface. The oxygen distance perpendicular to the surface in ${\mathrm{Li}}_n\mathrm{O}$ is optimized with respect to the cluster total energy. The changes of various properties of the adsorption of O on the ${\mathrm{Li}}_n$ clusters, in particular binding energy and equilibrium distance from the surface, are considered as a function of the number of atoms, $n$, in the substrate cluster. The differences among these properties for the different adsorption sites are rather large and much larger than the changes obtained for the same site with different size ${\mathrm{Li}}_n$ clusters. The Li-O bond is always highly ionic. In the on-top position the oxygen becomes stable as ${\mathrm{O}}^{-}$ with a binding energy $D\approx 1.5$ eV. In the central position the oxygen stabilizes in the surface plane as ${\mathrm{O}}^{2-}$ with $D\approx 3$ eV. In the bridge position the oxygen penetrates as ${\mathrm{O}}^{2-}$ into the substrate with $D\approx 5-6$ eV. The results indicate that upon adsorption in the bridge position the oxygen penetrates into the lithium (100) surface and stabilizes as ${\mathrm{O}}^{2-}$ with a high binding energy. This gives a reasonable first step of the metal oxidation which in the real situation happens spontaneously. The energy of the oxygen vibrations normal to the surface is similar for the on-top and central positions ($\approx 53$ meV), whereas the value for the bridge position is much smaller ($\approx 29$ meV). This difference can be understood on the basis of simple geometric arguments.